As people's interest in whitening their teeth increases, a number of tray products providing teeth whitening in a short period of time have become commercially available. A dentist-prescribed at-home bleaching product using a tray customized to fit a wearer is intended for overnight use for 2 weeks. However, since the wearer must keep his mouth closed while asleep for 2 weeks, the product is very inconvenient to use. For this reason, dentists have said that people prefer in-office bleaching products capable of providing whitening in only one dental treatment. In order to shorten the 2-week period to a single dental treatment, hydrogen peroxide at a concentration as high as 30˜35% is used. In this case, an element such as a rubber dam must be used to prevent contact of the whitening product with body parts other than the teeth. As another technique, it is known that hydrogen peroxide at a concentration of about 16% can be combined with the use of laser, heat activator or photo activator to exert synergistic effects. The in-office bleaching product of this type has disadvantages of severe irritation and earlier retention due to the use of hydrogen peroxide at such a high concentration. In addition, although there has been considerable interest in in-office bleaching products and at-home bleaching products, these products are disadvantageous in that they are expensive and a wearer needs to regularly visit a dentist.
Accordingly, there is a need for over-the-counter (OTC) products for teeth whitening which exhibit whitening properties comparable to dental treatment without the aid of a dentist at low cost. A number of teeth whitening products using trays for OTC products are now available at low cost in supermarkets and drugstores, but are known to have many problems. For example, since the OTC products use one-sized and single-shaped trays, they do not fit various sizes and shapes of users' teeth, which causes irritation to the teeth due to poor fitting trays and gels excessively applied onto the trays. In addition, since the leakage of gel may take place in use, there are problems in terms of convenience and safety. Further, since most of the OTC products are highly noticeable, it is difficult to use them on a daily basis. Furthermore, the users of OTC products must keep their mouth closed for a long time during use.
In order to solve these problems of conventional teeth whitening products using trays, Japanese Patent Laid-open No. Hei 10-017,448, assigned to Lion Corp., discloses a plaster in a sheet form for the oral cavity which comprises a teeth adhesive layer and a supporting layer. Examples of whitening agents which can be used in the plaster include kojic acid and derivatives thereof, ascorbic acid and derivatives thereof, carbamide peroxide and the like. Among them, kojic acid and various salts thereof are particularly effective. However, since the above-mentioned whitening agents are strong acids, they may cause irritation of the oral cavity due to their low pH. Such agents provide substantial whitening at a high acidity. However, it is difficult to obtain a plaster with good whitening without causing any irritation.
U.S. Pat. No. 5,310,563 (“Curtis et al.”) suggested a strip for teeth whitening which comprises a capsule obtained by encapsulating peroxide or various active ingredients in a putty-like material using a silicon polymer. The adhesion of the strip for teeth whitening to the teeth results from the elasticity of the putty such as rubber. However, meaningful attention is not given to the shape of the strip. The strip has a drawback that since the peroxide is encapsulated in the putty-like material, the whitening properties are relatively poor although the strip is attached to the teeth for the same time period. Further, since the adhesion between the strip and the teeth depends on the elastic strength, some stimuli such as coughing or collision with something in a mouth such as a tongue may separate the strip from the teeth.
On the other hand, U.S. Pat. No. 6,435,873 discloses a strip for teeth whitening using a gel retention insert without the use of a tray. The gel retention insert is a product that was developed in order to further enhance the adhesion between gel and the teeth, and has a microstructure. The gel retention insert has a microstructure in which a PSA (pressure sensitive adhesive) is used and teeth whitening ingredients are dispersed. However, the teeth whitening effects of the gel retention insert are not verified. Since typically polymers used as PSAs employ solvents unsuitable for oral products, e.g., methylene chloride or ethylacetate, etc, instead of water or alcohol, they are considered to be unsuitable to directly attach to the teeth in the oral cavity. In addition, in the case of an acrylic polymer (a wet PSA) which has a low compatibility with a peroxide, the resulting strip for teeth whitening is thought to be inappropriate for commercialization.
U.S. Pat. No. 6,419,906, issued to Colgate, discloses a strip for teeth whitening comprising a thermoplastic ethylene oxide polymer and a solid percarbonate as a teeth whitening agent. However, since the ethylene oxide polymer has poor adhesion to the teeth, it is believed that the wearability of the strip is poor.
In particular, teeth whitening strips were commercialized by P&G. The teeth whitening strip is manufactured by uniformly and thinly coating teeth whitening ingredients onto a thin and flexible polyethylene strip without the use of a tray and, thus, problems of conventional tray products can be avoided. However, since the teeth whitening strip is a wet type strip, the adhesive strength of the strip to the teeth is not so strong. Accordingly, the strip can be easily attached and peeled off, but has a drawback in that when coughing and laughing out loud, it does not remain attached at a desired position for a desired period of time and can be easily detached from teeth. Since the strip for teeth whitening has poor adhesion to the teeth, a wearer may also feel an unnatural sensation. In addition, since the teeth whitening strip is not closely fixed to the teeth, a wearer feels that the strip is loose and cannot continuously wear the strip for 30 minutes or more. According to one study from P&G, the concentration of hydrogen peroxide as a whitening agent contained in the wet type product sharply decreases 30 minutes after adhesion, and thereafter does not contribute to whitening effects. A recently published U.S. patent application Ser. No. 2003/0219389 discloses tooth whitening products. According to this patent application publication, the concentration of a whitening agent in a tooth whitening substance is a main factor among many factors capable of enhancing whitening effects. The patent publication also mentions that hydrogen peroxide having a concentration of 7.5% or more and a density of about 1.3 mg/cm2 or less in a tooth whitening composition causes a tolerable degree of irritation. However, the degree of irritation is dependent on individuals. In addition, since it is well known that high concentration hydrogen peroxide involves severe irritation, improvement in whitening properties using low concentration hydrogen peroxide is preferred. In particular, when the wet type products from P&G are touched by the hands or contact the skin, the whitening agent begins to be released since the wet type gel is already hydrated. As a result, discoloration of the hands or skin occurs even by a product containing 5.3% hydrogen peroxide.
In conclusion, since high concentration hydrogen peroxide involves severe irritation, it is desirable to provide products and methods that improve teeth whitening, convenience in use, and safety without increasing the concentration of hydrogen peroxide.